The present invention relates to a method of implementing the power transmission of a forest machine.
The invention also relates to an arrangement for implementing the power transmission of a forest machine.
In the power transmission of moving working machines, which in this context refer particularly to forest machines, a plurality of different power transmission means have been used as the technology has developed. In the case of forest machines, such power transmission may be considered to be divided into two different main classes on the basis of the purpose of use; on the one hand to the movement of the working machine itself, so-called propulsion, the required driving power transmission and, on the other hand, to the power transmission of the actuators comprised by the working machine, such as chargers, hoisters, felling heads and the like.
From the starting phases of mechanical harvesting, completely mechanical driving power transmission solutions are known that are based on different switches and mechanical transmissions, which enable a crude control of the direction of movement and speed of the vehicle. In such earlier forest machines, the loading of pieces of timber, such as logs, took place manually or by means of mechanical lifting aids operating via wires, such as lifting boom systems based on a winch.
In fact, a significant breakthrough in the field of forest machine technology was the realization of applying pressure medium-transmitted power transmission, which in this context refers generally to hydraulic power transmission in an apparatus employed for the loading and processing of timber. As a result of this realization, a so-called hydraulic knuckle boom charger was generated. In fact, hydraulic power transmission has established its position as the most common power transmission method in boom structures of moving working machines. The use of hydraulic technology in these applications is supported by its excellent power density, the flexible ways of placement thereof in moving structures and the feasibility to easily accomplish strong forces in a manageable and controllable manner.
For the implementation of stepless speed regulation in driving power transmission, in turn, hydrodynamic power transmission arrangements provided with a so-called torque converter have been developed and are known. In addition to an actual torque converter, the power transmission of a working machine may also be implemented with automatic transmission, practically composed of both a part similar to the torque converter and a transmission operating steplessly at more than one transmission ratios.
Power transmission solutions based on a torque converter are still in use in the transmission of high powers, for example in forest machines that are used for skidding whole tree trunks. Furthermore, hydrodynamic power transmission is still common in mining vehicles, such as dumpers, for example.
However, in forest machines utilizing a so-called CTL (Cut-to-Length) method, a change over to the use of chiefly hydrostatic power transmission in driving power transmission occurred years ago because of its better maneuverability and favourable braking characteristics. Hydrostatically implemented power transmission can also be more flexibly mounted into the structure of the machine than the above-described hydrodynamic power transmission. The implementation of such hydrostatic driving power transmission may be composed of for instance wheel-specific hydraulic engines or a hydrostatic/mechanic concept, wherein hydrostatic power transmission, typically composed of a hydraulic pump and a hydraulic engine, of which at least one has usually an adjustable rotational volume, is arranged between the primary power source of the working machine and the conventional, mechanical power transmission.
However, a drawback of hydrostatic power transmission can be considered its poor efficiency, particularly at high driving speeds. The efficiency also varies significantly depending on the loading situation.
Lately, manners of electrical implementation of the driving power transmission of various working machines have been surveyed, whereby the driving power transmission could be implemented either partly or even entirely electrically. The interest in electrical power transmission has increased particularly because of its favourable efficiency and the feasibility of using electric motors for braking and in this connection even for using them for energy recovery. Accordingly, heavy working machines, such as mining and quarry vehicles, for example, are known, whose driving power transmission is implemented entirely electrically. On the other hand, hybrid solutions of passenger cars, for example, are also known, wherein the effect of the driving power transmission is achieved partly by means of an electric motor and partly by means of a conventional combustion engine. An example of such an arrangement is set forth in patent EP1034968.
However, when forest machines are studied, the replacement of hydrostatic driving power transmission with electrical power transmission does not necessarily result in an important improvement in the overall efficiency of the forest machine. It does not either provide other particularly important advantages in the operating environment of the forest machine. In practice, such a driving power transmission solution would require at least two parallel, high-capacity power transmission systems in the forest machine.
In the case of forest machines in particular, the problems caused by electric power transmission in the forest machine environment result first and foremost from the momentary requirements of the various boom systems, hoisters, loaders, felling devices and other timber-processing devices for very high effects from the primary power source of the working machine. This way the primary power source is found to operate in a situation wherein the power source, dimensioned according to the maximum power required by the forest machine, is loaded very unevenly. During part of the operating time of the forest machine the power source is loaded even with a significantly slight partial load.
The above-mentioned functions of the various boom systems, hoisters, loaders, felling devices and other timber-processing devices of the forest machine are, however, extremely difficult to implement without pressure medium-transmitted hydraulic power transmission. In these applications, the advantage of hydraulic power transmission in particular is its extreme power density and good adjustability. Furthermore, the above-mentioned, hydraulically implemented functions are most frequently very critical as regards the performance and productivity of the forest machine. For example, when a forwarder is studied, the loader therein is in use for a significant portion of the working time, thus making high demands on the power source.